Conventionally, an optical pickup lens device is used under conditions in which the wavelength of a light source is 650 nm or more and the numerical aperture of an objective lens element is such that NA=on the order of 0.6; therefore the amount of displacement of a spot caused by axial chromatic aberration or magnification chromatic aberration is not likely to cause problems.
In recent years, however, along with an increase in the capacity of information recording media, there have been developments in the shortening of the wavelength of a light source and the increase of NA (numerical aperture) in an optical information recording device. In such a short-wavelength region, since the dispersion of an optical material such as a lens element is very large, a slight change in the wavelength of a bundle of rays significantly changes the refractive index of the optical material. Accordingly, in recent optical pickup lens devices, it is necessary to consider chromatic aberration correction.
Particularly, in an optical information recording device of DVD±RW, etc., which are currently widely used in a DVD recorder, etc., since the recording and erasing of information are performed using a phase change in a medium, the optical power used when writing or erasing information is made different from the optical power used when reading written information. Thus, in an optical information recording device using a phase-transition medium, it is not possible, in principle, to avoid the wavelengths of a bundle of rays emitted by the light source from significantly changing upon switching between recording or erasing and reproduction.
Hence, in an optical information recording device using a phase-transition medium, chromatic aberration correction in an optical pickup lens device is a critical problem for the following reason. In an optical pickup device using a phase-transition medium, if the chromatic aberration in the lens device is not corrected, a steep focus position change occurs due to the change in wavelength emitted by the light source, and consequently, focus control may not be performed.
Conventionally, in order to suppress such chromatic aberration, as described in Japanese Laid-Open Patent Publications No. 64-19316, No. 7-294707, and No. 11-337818, there have been proposed techniques such as a technique of allowing an objective lens element to have chromatic aberration correction functionality, a technique of allowing a collimating lens disposed between a light source and an objective lens element to have chromatic aberration correction functionality, a technique of additionally inserting a chromatic aberration correcting element in an optical path to excessively correct chromatic aberration, and thereby canceling out chromatic aberration in an objective lens element.